Обсуждение: "select max/count(id)" not using index

Hi.

I have a table with 24k records and btree index on column 'id'. Is this
normal, that 'select max(id)' or 'select count(id)' causes a sequential
scan? It takes over 24 seconds (on a pretty fast machine):

=> explain ANALYZE select max(id) from ogloszenia;
                            QUERY PLAN
----------------------------------------------------------------------
 Aggregate  (cost=3511.05..3511.05 rows=1 width=4) (actual
time=24834.629..24834.629 rows=1 loops=1)
   ->  Seq Scan on ogloszenia  (cost=0.00..3473.04 rows=15204 width=4)
(actual time=0.013..24808.377 rows=16873 loops=1)
 Total runtime: 24897.897 ms

Maybe it's caused by a number of varchar fields in this table? However,
'id' column is 'integer' and is primary key.

Clustering table on index created on 'id' makes such a queries
many faster, but they still use a sequential scan.

Richard.

--
"First they ignore you. Then they laugh at you. Then they
fight you. Then you win." - Mohandas Gandhi.

> I have a table with 24k records and btree index on column 'id'. Is this
> normal, that 'select max(id)' or 'select count(id)' causes a sequential
> scan? It takes over 24 seconds (on a pretty fast machine):
>
> => explain ANALYZE select max(id) from ogloszenia;

Yes, it is.  It is a known issue with Postgres's extensible operator
architecture.

The work around is to have an index on the id column and do this instead:

SELECT id FROM ogloszenia ORDER BY id DESC LIMIT 1;

Which will be really fast.

Chris

Hello

It is normal behavior PostgreSQL. Use

SELECT id FROM tabulka ORDER BY id DESC LIMIT 1;

regards
Pavel

On Mon, 22 Dec 2003, Ryszard Lach wrote:

> Hi.
>
> I have a table with 24k records and btree index on column 'id'. Is this
> normal, that 'select max(id)' or 'select count(id)' causes a sequential
> scan? It takes over 24 seconds (on a pretty fast machine):
>
> => explain ANALYZE select max(id) from ogloszenia;
>                             QUERY PLAN
> ----------------------------------------------------------------------
>  Aggregate  (cost=3511.05..3511.05 rows=1 width=4) (actual
> time=24834.629..24834.629 rows=1 loops=1)
>    ->  Seq Scan on ogloszenia  (cost=0.00..3473.04 rows=15204 width=4)
> (actual time=0.013..24808.377 rows=16873 loops=1)
>  Total runtime: 24897.897 ms
>
> Maybe it's caused by a number of varchar fields in this table? However,
> 'id' column is 'integer' and is primary key.
>
> Clustering table on index created on 'id' makes such a queries
> many faster, but they still use a sequential scan.
>
> Richard.
>
>

Dnia 2003-12-22 11:39, Użytkownik Ryszard Lach napisał:

> Hi.
>
> I have a table with 24k records and btree index on column 'id'. Is this
> normal, that 'select max(id)' or 'select count(id)' causes a sequential
> scan? It takes over 24 seconds (on a pretty fast machine):
'select count(id)'
Yes, this is normal. Because of MVCC all rows must be checked and
Postgres doesn't cache count(*) like Mysql.

'select max(id)'
This is also normal, but try to change this query into:
select id from some_table order by id desc limit 1;

What is your Postgresql version?

Regards,
Tomasz Myrta

Guten Tag Ryszard Lach,

Am Montag, 22. Dezember 2003 um 11:39 schrieben Sie:

RL> Hi.

RL> I have a table with 24k records and btree index on column 'id'. Is this
RL> normal, that 'select max(id)' or 'select count(id)' causes a sequential
RL> scan? It takes over 24 seconds (on a pretty fast machine):

Yes, that was occasionally discussed on the mailinglists. For the
max(id) you can use instead "SELECT id FROM table ORDER BY id DESC
LIMIT 1"


Christoph Nelles


=>> explain ANALYZE select max(id) from ogloszenia;
RL>                             QUERY PLAN
RL> ----------------------------------------------------------------------
RL>  Aggregate  (cost=3511.05..3511.05 rows=1 width=4) (actual
RL> time=24834.629..24834.629 rows=1 loops=1)
RL>    ->  Seq Scan on ogloszenia  (cost=0.00..3473.04 rows=15204 width=4)
RL> (actual time=0.013..24808.377 rows=16873 loops=1)
RL>  Total runtime: 24897.897 ms

RL> Maybe it's caused by a number of varchar fields in this table? However,
RL> 'id' column is 'integer' and is primary key.

RL> Clustering table on index created on 'id' makes such a queries
RL> many faster, but they still use a sequential scan.

RL> Richard.




--
Mit freundlichen Grüssen
Evil Azrael                            mailto:evilazrael@evilazrael.de